1. Field of the Invention
The present invention relates to a recording apparatus utilizing a laser beam, and more particularly to a recording apparatus for recording information on a photosensitive member with a laser beam emitted by a semiconductor laser element.
2. Description of the Prior Art
The use of a semiconductor laser element as a light source is already well known in the fields of optical communication and of recording apparatus as disclosed in the U.S. patent application Ser. No. 822,340 now abandoned in favor of continuation application Ser. No. 87,705, filed Oct. 24, 1979 of the present applicant. In such applications there is commonly employed an automatic power control method (hereinafter called APC) for maintaining a constant optical output of the semiconductor laser.
FIG. 1 shows the process of such automatic power control in which the optical output from a semiconductor laser element 1 is detected by a photodetector 2 and, after amplification by an amplifier 3, is supplied as a feedback signal to a laser drive circuit 4 in such a manner as to maintain constant the output of said photodetector 2. Such control is widely employed to compensate for the change in the output of the semiconductor laser element resulting from the change in the circumferential conditions or from the deterioration of said element.
However, in the application of a semiconductor laser element in a recording apparatus utilizing the electrostatic recording process, the output of photo-sensitive member made, for example, of cadmium selenide or selenium, is dependent on the wavelength. In the case of a short-wavelength laser such as an ordinary He-Ne laser, the conventional APC is applicable since such photosensitive members have a relatively flat spectral sensitivity in the range of the laser wavelength.
In the near-infrared region (approximately 8000 A) of the semiconductor laser element, the ordinary photosensitive members for electrostatic recording have a relatively low sensitivity and are therefore often sensitized in case a semiconductor laser element is to be utilized in such image recording apparatus. However, in consideration of the stability of image quality and of the photosensitive member, the sensitivity after said sensitization cannot be made flat in the wavelength region of the semiconductor laser and shows an inevitable dependence on the wavelength as shown by curve a in FIG. 2, thus becoming lower at a longer wavelength.
For this reason it becomes necessary to control the optical output in relation to the wavelength of the semiconductor laser element to be employed, and the conventional APC method is unable to ensure a satisfactory image quality in case of wavelength fluctuation during the function of the semiconductor laser element.
As an example, the output wavelength of an ordinary semiconductor laser element shows a fluctuation of 2.5 to 3.0 A per degree so that it shows a maximum change of 90 A for a temperature change of 30.degree. C. Consequently the obtainable image quality becomes more deteriorated as the wavelength dependence of the sensitivity is higher.